Collaborative Research: GEM--Whistler-mode Wave Properties in Plasmasphere-Plasmatrough Transition Regions

Both chorus and hiss waves can directly impact the Earth’s radiation belt environment, from accelerating particles up to relativistic energies, which can subsequently damage both humans and hardware in space, to precipitating particles into the atmosphere where they pose a potential danger to aircraft and passengers on polar routes. It is often assumed that chorus and hiss are well-separated by a sharp density gradient boundary called the plasmapause, with all hiss occurring on one side of the plasmapause and all chorus occurring on the other. This assumption has a problem: around half of the time, the plasmapause boundary is ill-defined, and is observed as less of a boundary, but more of a transition region. When this occurs, chorus and hiss can co-occur and overlap spatially, meaning particles may feel the combined effect of both wave types simultaneously. This project will characterize wave properties in these transition regions and establish the impact of the transition region on radiation belt simulations. This project will also support students and researchers from a teaching-focused college to actively participate in geospace science research. The investigation will first focus on cases where the plasmapause density gradient is not sharp (e.g., spans up to several L-shell), seeking to determine whistler-mode wave properties in the plasmasphere-plasmatrough transition region when a well-defined plasmapause may not exist (i.e., soft plasmapause). Data collected during these intervals are often excluded from statistical studies, which has led to wave property databases being built only from time periods when the plasmapause is a sharp, well-defined density gradient (i.e., hard plasmapause). This usually occurs during elevated geomagnetic activity, when waves are more intense, thus biasing predictive radiation belt models towards stronger wave-particle interaction processes. The team will construct full statistical hiss and chorus wave models for both the ‘hard’ and ‘soft’ plasmapause cases. This will be the first time that the characteristics and role of whistler-mode waves in spatially broad plasmasphere-plasmatrough transition regions has been directly investigated. The results will supply radiation belt modelers with the critical information of wave properties in the often-excluded case of ‘soft’ plasmapause transition regions (which account for ~50% of occurrences). This will improve the dynamical determination of acceleration and scattering regions in the radiation belts by removing bias towards plasmapauses defined by steep density gradients, and the extreme separation of hiss and chorus. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.